Chronic lymphocytic leukemia (CLL) is the most common adult leukemia in the United States and Europe. Recent progress in our understanding of CLL biology and the development of new therapy such as fludarabine-based regimens has led to improvements of the therapeutic outcomes. However, many CLL patients, particularly those with loss of p53 due to chromosome 17p deletion or p53 mutations, are highly resistant to the current therapeutic agents and have very poor prognosis. The underlying mechanisms responsible for the poor therapeutic responses of CLL cells lacking p53 are poorly understood, and effective therapies to treat this population of CLL patients remain to be developed. The current research proposal aims to answer some of the key questions in this area, and to develop novel therapeutic strategies for treating refractory CLL. Our previous studies showed that CLL cells from patients in advanced disease stages exhibit mitochondrial DNA (mtDNA) mutations and dysfunction, elevated generation of reactive oxygen species (ROS), and decreased sensitivity to fludarabine. Furthermore, p53 plays an important role in maintaining mitochondrial genetic integrity through its interaction with mitochondrial pol 3. Importantly, we observed that cancer cells with increased ROS are highly sensitive to further ROS stress induced by a natural compound 2-phenethyl isothiocyanate (PEITC). Our preliminary studies have revealed promising activity of PEITC against refractory CLL cells, leading to major efforts to develop this compound for clinical treatment of CLL. Based on these observations, we hypothesize that CLL cells with p53 deletion are prone to develop mitochondrial mutations and dysfunction leading to a decrease in apoptotic response and resistance to conventional therapeutic agents, but remain sensitive to PEITC which kills drug-resistant CLL cells through ROS-mediated mechanism. This research project will investigate 3 specific aims. (1) We will use primary leukemia cells from CLL patients with different p53 status compare their mtDNA mutation rates, and use a novel CLL culture system to test the role p53 deletion in mitochondrial mutations and attenuation of drug apoptotic response to therapeutic agents. (2) Investigate the ability and mechanisms of PEITC and other ROS-stressing agents to effectively kill primary CLL cells that lack p53 and are resistant to conventional drugs. (3) Test the in vivo therapeutic activity of PEITC, alone or in combination with other drugs, using mouse xenograft models bearing human CLL cells with or without p53 to evaluate if PEITC and other ROS-stressing agents can induce CLL remission and improve survival. This study will reveal the novel roles of p53 in affecting mitochondrial genetic stability and function and drug sensitivity in CLL cells, and provide new strategies to effectively kill refractory CLL cells through ROS-mediated mechanism. Because PEITC is a natural compound found in vegetables with low toxicity to normal cells, its applications for clinical treatment of refractory CLL is feasible and highly significant.